First of all, let's be clear about what's meant by "posters" and "covers" in the context of videos. A cover is a the entire cover (i.e., front, side and back) of a DVD case. Technically, a poster is a poster that promotes the movie, and has nothing to do with DVD media (which normally would not exist at the time the poster is created). As a practical matter, what we refer to as a poster is most often just the front cover of a DVD. Annoying, much of what is available are images of the actual front of a DVD case, rather than just the art (the "poster" or "front cover") itself. The obvious reason for this is that the largest sources of "posters" are in the business of selling DVD's (e.g., amazon).
Posters are easier to work with than covers, mainly because the plugins that retrieve them are more or less automatic. Since covers are for specific DVD editions, the databases that provide them are geared more toward identifying the edition than just offering the best art for the particular movie. The quality of covers is generally better than posters. If I can't find a good quality poster, I'll download a cover and crop the front to make a poster.
PVD cannot automatically import folder.jpg's saved in movie directories. With some effort, you could probably adapt the process discussed
here to import them. If the images you have are not of consistently good quality, it may be easier just to use PVD's poster plugins. In cases where you're sure what you've already got is better than what the plugin downloaded, you can manually load your folder.jpg image.
In using PvdImport, you have the choice of whether or not to import PVD posters, and, if so, how to handle them. I'm not sure how MC decides whether to use a poster so imported, or an existing folder.jpg. My guess is it uses folder.jpg if something like title.jpg is not found in the File or Cover Art directories.
